1) Field
One or more embodiments of the present invention relate to a transition metal/carbon nanotube composite, a method of preparing the same, an electrode including the transition metal/carbon nanotube composite, a capacitive deionization device including the electrode and an electric double layer capacitor including the electrode.
2) Description of the Related Art
Capacitive deionization (“CDI”) is a technique for removing an ionic material in a medium by absorbing the ionic material into nano-sized pores of a surface of a carbon electrode. More specifically, in CDI, a first voltage is applied to the carbon electrode to absorb the ionic material therein, while a second voltage having an opposite polarity to the first voltage is applied to the carbon electrode to regenerate the carbon electrode after the ionic material is absorbed by the carbon electrode and removed from the medium. The absorbed ionic material is then discharged from the carbon electrode with water. As a result, CDI does not require chemicals to regenerate the carbon electrode, nor does CDI require an ion exchange resin and/or expensive filter or membrane. Also, in CDI, hard constituent materials and harmful ions are removed from the carbon electrode, and an insulation function, e.g., a removal function, and a capacitance of the carbon electrode are thereby improved.
In CDI, when a direct current (“DC”) voltage having a relatively low potential difference is applied to the carbon electrode while water containing dissolved ions flows through a flow path including the carbon electrode therein, anions of the dissolved ions are absorbed and concentrated at an anode, while cations of the dissolved ions are absorbed and concentrated at a cathode. Accordingly, when application of the DC voltage to the carbon electrode is stopped, the anions and cations, concentrated at the anode and the cathode, respectively, are desorbed from the carbon electrode.
The carbon electrode is typically manufactured by binding an activated carbon having a polytetrafluoroethlyene (“PTFE”) binder to ensure that the carbon electrode has sufficiently low electrode resistance and large specific surface area. Alternatively, the carbon electrode may be manufactured by carbonizing a resorcinol formaldehyde resin and then performing a complicated drying process to manufacture a carbon electrode having a plate shape.
However, when a general carbon active material is used to manufacture the carbon electrode, a capacitance of the carbon electrode is substantially limited.